Vaisala HMP228 - Manual (Page 97)

HMP228
M210282en-A Appendix 5: Digital current loop module
91
The digital current loop module has separate lines for transmitting and receiv-
ing. Both single loop wiring and dual loop wiring can be used (see figure).
Dual loop connection makes it possible to have a few more transmitters on the
same loop pair. A single loop connection has simpler wiring. Data transmis-
sion is achieved by switching the loop current on and off.
Normally, current flows through the loop(s) even when the HMP228 transmit-
ter is not on, so switching one transmitter off does not affect the other trans-
mitters on the loop.
When the wires have been connected correctly, the voltage drop from RX+ to
RX- is below 2 V. If the wires RX+ and RX- or TX+ and TX- are connected
incorrectly, the voltage drop from RX+ to RX- or from TX+ to TX- is below 1
V and the transmitter does not work. Even then the current goes through the
loop and the other transmitters can be operated normally.
When the loop supply is current limited, the data lines can withstand short cir-
cuit to ground and to each other. They do not survive connection of supply
voltage to the data lines.
The module must be mounted on the main board in the right direction. It can
be mounted in the wrong direction or to the wrong pins without breaking the
module; it simply does not work then. Reverse wiring of RX+ and RX- or
TX+ and TX- does not affect the module.